1. Field of the Invention
The present invention relates to disk drive systems of the type having a plurality of concentric data tracks located on a rotatable disk. More particularly, the present invention relates to a system for accurately positioning a magnetic head with respect to a desired reference track. Still more particularly, the present invention relates the provision of a track locating system in a low cost disk drive which is compatible with external controllers.
2. Description of the Prior Art
Prior art low cost disk drive systems typically employ an open loop positioning system for locating a magnetic transducer, or head, with respect to the data tracks on a disk. In one common system, a stepper motor is used to position the head with respect to the data tracks, and the number of steps of the motor with respect to a reference position is monitored to determine the location of the head. An optical sensor senses a position flag coupled to the drive capstan of the motor to provide an indication of when the head is at a reference track, arbitrarily designated track zero. Whenever power is applied to the disk drive system, the head position is initially unknown. The motor is stepped until the opitical transducer provides an indication that the motor has positioned the head at track zero. A microprocessor is employed to keep track of the number of subsequent steps taken by the motor to determine head position with respect to the reference track.
The prior art mechanical system described above has several disadvantages associated with it. Primary among these is that the optical sensor and accompanying components are quite expensive. Furthermore, the sensor requires a critical adjustment in order to ensure that it accurately indicates when the head is positioned at track zero. It is an object of the present invention to eliminate the need for an expensive optical sensor and associated components. It is a further object of the invention to eliminate any mechanical components which require critical adjustments in order to provide accurate track zero information.
In the system described above, the entire disk surface is available for writing data. An index pulse is generated once per revolution of the disk, and an external controller allows data to be written on a selected track of the disk between successive index pulses. More specifically, the controller writes a predetermined amount of serial trasmitted data between successive index pulses. The rate at which the data is transmitted is dictated by industry standard and, hence, the amount of data to be written between successive index pulses to the controler is predicated by the nominal psuedo-standard disk speed of 3600 revolutions per minute. It is permissible and commonly practiced within the disk drive industry to alter slightly the disk speed relative to 3600 revolutions per minute and hence the overall amount of data to be written.
More complex disk drive systems often employ servo information located on the disk surface to control track seeking and track following (centering) operations. Often, the disk includes a plurality of servo sectors, and each data track includes a unique address code which is read to determine the position of the head. Such systems are disclosed in U.S. Pat. Nos, 4,195,320 to Andresen, 4,163,265 to Van Herk et al., 4,151,571 to Cardot et al., 4,048,660 to Dennison et al. (a unique track address for each track within a six track region) and 4,354,210 to Droux. Such systems require relatively complicated decoding circuits in order to determine the track number. In addition, the format of data on the disk must be predetermined due to the existance of multiple servo sectors.
Additonal disk drive systems which include track locating systems are disclosed in U.S. Pat. Nos. 4,405,956 to Marshall, 4,390,912 to Hertrich et al and 4,396,959 to Harrison et al. In Marshall, two pairs of positive and negative servo marks are recorded in a plurality of servo sectors. The servo marks are monitored to provide tracking signals which are ninety degrees out of phase, and trigonometric identities are employed to determine track location. In Hertrich, alternating positive and negative servo bursts in sectors in data tracks are employed to determine track location. In addition, constant level servo bursts are recorded in inner and outer guardbands adjacent to the data tracks to indicate when the head is outside of the data tracks. In Harrison, a single sector of servo information is provided on the disk and controls track centering functions. Track locating is accomplished by an encoder coupled to the head positioner motor, with zero crossings of the encoder corresponding to various data tracks. In the Harrison patent, the single servo sector is masked from the user by reducing the rotational speed of the disk drive. This maintains the standard writing time period and thus enables the data format to be selected by means of an external controller. The servo sector does not contain any track zero information. Rather, this information is derived from the external position transducer coupled to the head actuator.